The invention relates to a vacuum pump with a stator on which a rotor with at least one radial magnetic bearing and an axial magnetic bearing is supported in a contactless manner.
In vacuum pumps, magnetic bearings are frequently used for bearing the rotor. Magnetic bearings permit high rotational speeds of the rotor and are lubricant-free, whereby a pollution of the vacuum side of the pump by lubricant is excluded. From U.S. Pat. No. 5,166,566, a vacuum pump is known where the radial bearings are each formed by permanent magnet rings arranged so as to be slightly axially offset with respect to each other. Since the magnetic rings repelling each other also repel each other in an axial direction due to the offset arrangement, the rotor is biased in an axial direction. At an axial end of the rotor, a large lifting magnet is arranged which has an attracting effect upon a rotor disk. The lifting magnet is controlled in such a manner that it keeps the rotor in the axially biased position. In this case, the lifting magnet always has to compensate the axial bias. From U.S. Pat. No. 5,059,092, an axial bearing is known where two rotor disks are axially magnetized so that the stator-side magnet coils are able to act upon the rotor in both axial directions, depending on the direction of the current flowing therethrough. The rotor-side magnet disks have an attracting effect upon the yoke iron of the magnet coil so that in both axial directions, considerable forces of axial attraction occur that pull the rotor out of its axial central position and have to be compensated by the magnet coils.
It is the object of the invention to improve the axial magnetic bearing of a vacuum pump.
This object is solved, according to the invention, with the features of claim 1.